Indiscriminate fertiliser use hurts the soil itself, turning it acidic and salty, suppressing the symbiotic relationships between fungi and plant roots, sometimes turning beneficial bacteria against each other. Long-term use of fertilisers risks turning even fertile soil to desert.
What can be done? One possible solution is being pursued by Carlos Monreal of Carleton University in Ottawa, Canada, and his colleagues. They realised that final price is the driver for farm products – we cannot go back to the natural rotation of crops without a big rise in food prices (and wretched biofuels in Iowa). He thinks the answer is to make fertilisers smarter. Monreal wants to exploit the way plants signal to bacteria by releasing chemicals. The plant tells the microbes they need nitrogen. The microbes then begin working to free nitrogen from organic matter, and the plant soaks it up. In 2011, after nearly a decade of sifting through hundreds of chemicals in soil samples taken from fields of wheat and canola (oilseed rape), Monreal’s team identified five compounds that spike just as the plants take in ammonia – these are the chemical signals plants exude to ask for nitrogen.
The trick behind a better fertiliser is to keep its payload locked up until it encounters a plant’s signalling compounds. Aptamers are short strands of DNA that bind to specific chemicals, much the way antibodies do. After training them to recognise the five compounds, the aptamers were used as scaffolding around a tiny parcel of fertiliser. In the presence of one of the plant signalling compounds, the aptamers would bind to it, rupturing the scaffolding and releasing its contents. The results are fertiliser-filled capsules that open up in response to the appetite wheat and canola have for nitrogen. The technique is undergoing greenhouse trials.
An alternative approach is to replace synthetic fertilisers and chemical pesticides with the soil’s own microbiome to maintain their fertility. The idea is to use a “universal recipe” of beneficial bacteria, mycorrhizal fungi and humus that adheres to plant roots and helps them extract nutrients. The mixture causes desert-like test plots to sprout oats and leguminous plants called vetches. The few plants that grow in the control plots, dosed with traditional pesticides and fertilisers, are small and stunted; those treated with the cocktail are not just healthy at the surface, but their roots grow strong and long enough to pierce dirt as hard as rock.
Meanwhile, UN Global Soil Map project is creating a real-time, highly detailed, digital repository of the condition of soils worldwide: its clay, silt and organic carbon levels, together with acidity and overall density. By 2019, researchers aim to have mapped soils worldwide down to 100 metres. with the results accessible to all.
The problem is to get the attention of governments and the public to the threat. For soils on the brink, it may already be too late. Several researchers are agitating for the creation of protected zones for endangered soils, although there has been little official movement on the issue so far. One problem is defining what these areas should conserve: areas where the greatest soil diversity is present? Or areas of pristine soils that could act as a future benchmark of quality? ( adapted from an article by Joshua Howgego, New Scientist, October 2015)
